It is proposed to study the synaptic connections of the neurons in the outer plexiform layer of the turtle retina by electron microscopy. Long series (100-500) of ultrathin sections will be examined, and profiles of interest identified in the inner nuclear layer and followed to their synaptic connections with identified photoreceptor cells. The different types of horizontal cells will be distinguished on cytological criteria and their synaptic relationships (chemical and electrical synapses) with other horizontal or bipolar cells will be evaluated. Amacrine-like profiles which have been seen in the outer plexiform layer will be followed in serial section to determine whether they originate from interplexiform cells. Golgi-impregnated bipolar cells that can be considered the morphological counterparts of physiologically identified on-center and off-center bipolar types will be sectioned and examined by electron microscopy for the morphology of their synaptic junctions. At the same time, it will be possible to study the contacts of the impregnated bipolar cells with the different spectral types of cones. Due to the poor preservation of Golgi-impregnated material, it will be necessary to examine these bipolar-photoreceptor contacts in greater detail by looking at serial sections of well-fixed, conventionally prepared material. Interactions between the different spectral types of cones will also be investigated by serial section electron microscopy of Golgi-impregnated and horseradish-peroxidase injected photoreceptors. The purpose of this study is to gain an understanding of the morphological basis for functional pathways originating at the outer plexiform layer. Furthermore, since spectral types of cones are readily identifiable in the turtle retina, valuable information concerning color coding at the first synaptic level will be attainable.